Soak the saffron strands in a little fish stock. Pick out the crabs, remove as much of the crab meat from the shells as you can. Add to the soup. Taste, add salt, freshly ground pepper ...

As it turns out, the crabs had probably crossed the Atlantic as stowaways in the ballast water on barge ships, and they were first spotted in 2012 in Spain’s Ebro Delta on the Mediterranean Sea.